This invention relates to de-inking processes that utilise polymeric materials to promote separation of the ink from pulped cellulosic material, and to novel polymeric materials which are particularly suitable for this use, but which also can be used in other processes.
Conventional de-inking processes comprise forming a pulp of recycled paper and separating a hydrophobic fraction containing ink from the pulp and recovering the resultant de-inked pulp. The separation of the hydrophobic fraction is promoted by including de-inking chemicals in the pulp.
There is a major commercial need to provide processes for deinking paper that are efficient, that do not degrade the paper that is being recycled, that are cost effective, convenient to operate, and environmentally acceptable.
Traditional deinking processes comprise pulping the recycled paper in a highly alkaline environment but this can cause yellowing of the cellulose fibers and the necessity to provide high alkalinity and peroxide is undesirable. Processes operated at lower pH values have been proposed, for instance in WO90/10749 and in WO93/21376.
Various additives for one or more of the processes have been proposed. For instance in GB 2178079 it is proposed to add an accelerator, and included amongst the accelerators that are mentioned are copolymers of (meth) acrylic acid and (meth) acrylic ester having molecular weight 1,000 to 10,000 and with a molar ratio of the acid to ester groups of from 1 to 49. An example in the specification shows that bad results are obtained with a copolymer of 1 mole potassium methacrylate and 2 moles methyl methacrylate molecular weight 40,000, compared to various copolymers having molecular weight below 10,000.
Other polymers which have been proposed for deinking include an emulsion of PVC or polymethyl methacrylate (Tsellyul Bum. Karton 1978, 4, 6 and Chem. Abs. 89:7865) melamine resins (U.S. Pat. No. 5,286,390), resin prepolymers (U.S. Pat. No. 5,073,234 and EP 163444) and polyolefine emulsions (Chem. Abs. 79:147582 and JP-B-47040881).
Another additive which is sometimes proposed for use is bentonite.
The ability to achieve satisfactory deinking often constitutes a restriction on the amount or type of recycled paper that can be utilised for the production of many grades of paper. For instance the use of newsprint which carries water-based flexographic ink can significantly reduce brightness after deinking by known treatments. Accordingly many deinking plants cannot effectively utilise even small amounts of flexo-printed paper. There is an urgent need for improved deinking processes, including deinking of flexo-printed paper.
A first aspect of the invention, as described below, relates to a novel de-inking process that uses an emulsion polymer to promote the separation of the hydrophobic fraction.
It is standard practice to make a polymer emulsion by oil-in-water emulsion polymerisation of a substantially water insoluble monomer blend comprising water insoluble hydrophobic monomer, such as ethyl acrylate, and some hydrophilic monomer that, at the polymerisation pH, is substantially non-ionised and is preferentially soluble in the monomer blend but which is preferentially soluble in water when ionised, at a higher or lower pH. For instance methacrylic acid dissolves in ethyl acrylate when it is present as free acid at acidic polymerisation pH values to allow formation of a copolymer of ethyl acrylate and methacrylic acid, but when the polymer is exposed to alkali (so that the monomer would then have ionised to an alkali soluble salt, and would have preferentially dissolved in aqueous alkali) the ionisation of the methacrylic acid units in the polymer to the corresponding methacrylic acid soluble salt units causes the polymer to become soluble.
It is well known to produce polymers of this type as viscosifiers, in that the emulsion has low viscosity but even a small amount of the emulsion (for instance 3% dry weight polymer) in aqueous alkali gives a very high viscosity as a result of dissolution of the polymer.
It is also known to include small amounts of cross-linking agents in the initial polymerisation mixture with the intention that the dissolution of the emulsion polymer particles will be restrained or prevented. However the amount of cross-linker that is used is always sufficiently low, in practice, that the emulsion still provides a high viscosity in water after pH adjustment. Typically, satisfactory viscosifying emulsion polymers of this type are such that an aqueous composition, formed by blending 3% (dry weight polymer) of the emulsion in water with acid or alkali to provide the solubilisation of the polymer, has a viscosity above 100,000 cps measured by Brookfield RVT Viscometer. Indeed, a 3% composition is frequently a gel. These soluble or highly swelling polymers are not useful for the deinking process.
A second aspect of the invention, as described below, relates to a novel emulsion polymer which can be used in the deinking process.